This invention relates to an alloy ingot for permanent magnet of rare earth metal-iron or rare earth metal-iron-boron having a crystalline structure excellent in magnetic properties, anisotropic permanent magnet powders of rare earth metal-iron-boron, a method for producing the ingot or powders, and a rare earth metal-iron permanent magnet.
Permanent magnet alloy ingots are generally produced by a metal mold casting method consisting in casting molten alloy in a metal mold. If the molten alloy is to be solidified by the metal mold casting method, it is the heat conduction through the casting mold that determines the rate of heat removal during the initial stage of the heat removal process for the molten alloy. However, as solidification proceeds, the heat conduction between the casting mold and the solidified phase or in the solidifying phase determines the rate of heat conduction. Even though the cooling capacity of the metal mold is improved, the inner portions of the ingot and those portions of the ingot in the vicinity of the casting mold are subjected to different cooling conditions. Such phenomenon is the more pronounced the thicker the ingot thickness. The result is that in the case of a larger difference between the cooling conditions in the inner portions of the ingot and those in the vicinity of the ingot surface, an .alpha.-Fe phase having a grain size of 10 to 100 .mu.m is left in the cast structure towards a higher residual magnetic flux density region in the magnet composition, while the rare earth metal rich phase surrounding the main phase is also increased in size. Since the .alpha.-Fe phase and the rare earth metal rich coarse-grained phase can be homogenized difficultly by heat treatment usually carried out at 900.degree. to 1200.degree. C. for several to tens of hours, the homogenization process in the magnet production process is prolonged with crystal grains being increased further in size. Besides, since the ensuing nitriding process is prolonged, nitrogen contents in the individual grains become non-uniform, thus affecting subsequent powder orientation and magnetic characteristics.
Although crystals having a short axis length of 0.1 to 100 .mu.m and a long axis length of 0.1 to 100 .mu.m are known to exist in the structure of the ingot produced by the above-mentioned metal mold casting method, the content of these crystals is minor and unable to influence the magnetic properties favorably. There has also been proposed a method for producing a rare earth metal magnet alloy comprising charging a rare earth metal element and cobalt and, if needed, iron, copper and zirconium into a crucible, melting the charged mass and allowing the molten mass to be solidified to have a thickness of 0.01 to 5 mm by, e.g. a strip casting system combined with a twin roll, a single roll, a twin belt or the like.
Although an ingot produced by this method has a composition more uniform than that obtained with the metal mold casting method, since the components of the feed material consist in the combination of rare earth metal, cobalt and occasionally iron, copper and zirconium, and the produced alloy is amorphous, the magnetic properties cannot be improved sufficiently by the above-mentioned strip casting method. In other words, production of the crystal permanent magnet alloy by the strip casting method has not been known to date.